CN113820340A

CN113820340A - Method for preparing a sample for a transmission electron microscope

Info

Publication number: CN113820340A
Application number: CN202110659873.3A
Authority: CN
Inventors: E·范科耶; N·博斯曼; P·卡罗兰
Original assignee: Interuniversitair Microelektronica Centrum vzw IMEC
Current assignee: Interuniversitair Microelektronica Centrum vzw IMEC
Priority date: 2020-06-12
Filing date: 2021-06-15
Publication date: 2021-12-21
Also published as: US11437217B2; US20210391144A1; EP3922752A1

Abstract

A substrate is provided that includes a patterned region on a surface thereof defined by a given topography. The substrate will be processed to obtain TEM samples in the form of slices of the substrate. According to the method of the present invention, a conformal layer of contrast material is deposited over the topography by depositing a layer of contrast material over a local target area of the substrate spaced from the patterned area. The material is deposited by Electron Beam Induced Deposition (EBID). Deposition parameters, thickness of a layer deposited in a target region and distance of the target region to the patterned region, such that a conformal layer of contrast material is formed on the topography of the patterned region. This is followed by the deposition of a protective layer, which does not destroy the topography in the patterned area, since the topography is protected by the conformal layer. TEM samples are prepared in a manner known in the art, for example by FIB. The conformal contrast layer provides good contrast with the protective layer, thereby allowing high quality TEM analysis.

Description

Method for preparing a sample for a transmission electron microscope

Technical Field

The present invention relates to the field of Transmission Electron Microscopy (TEM), and in particular to a method for preparing a TEM sample to visualize nanoscale structures as produced in semiconductor processing.

Background

Transmission electron microscopes are widely used in the semiconductor industry to view the finest details of transistor and memory structures, down to the atomic level. One of the difficult steps is transmission electron microscope sample preparation. This is done in a focused ion beam milling (FIB) tool, whereby flakes of the order of tens of nanometers are taken from the sample under investigation. The sample slice needs to be thin enough to show electron transparency. In order to protect the structure itself from milling during the preparation of the lamellae, a mask and a protective layer are required. The protective layer may be applied by various methods: spin coating, physical vapor deposition, chemical vapor deposition, evaporation, or a combination of two or more of these methods applied sequentially. However, samples with a polymer top surface (e.g., including a patterned polymer photoresist layer) are susceptible to damage by any of the methods described above, or these samples do not exhibit sufficient contrast with the protective layer to be indistinguishable during transmission electron microscope observation. In the case of fragile structures such as polymer resist lines or porous silicon structures, it is not an option to deposit the additional contrast layer by sputtering techniques, which will destroy the structure.

Disclosure of Invention

It is an object of the present invention to provide a solution to the above problems. This object is achieved by the method disclosed in the appended claims. A substrate is provided that includes a patterned region defined by a given topography of nanometer-scale features (e.g., a set of parallel lines of polymer resist) on a surface of the substrate. The structure will be processed to obtain TEM samples in the form of substrate slices cut transversely to the substrate surface, aiming at visualizing the topography by TEM. According to the method of the present invention, a conformal layer of thin contrast material is deposited over the topography by depositing a thicker layer of contrast material over a local target region of the substrate that is spaced apart from (i.e. located at a non-zero distance from) the patterned region. The material deposited on the target area is deposited by Electron Beam Induced Deposition (EBID), i.e. no mask is used to cover the substrate surface outside the local target area. By a reasonable choice of the thickness of the layer deposited in the target region and the distance of said target region to the patterned region, a conformal layer of contrast material is formed over the topography of the patterned region, i.e. a layer that follows the topography but does not fill the spaces between adjacent features of the topography. This is followed by the deposition of a protective layer, which does not destroy the topography in the patterned area, since the topography is protected by the conformal layer. TEM samples are prepared in a manner known in the art, for example by FIB. The conformal contrast layer provides good contrast with the protective layer, thereby allowing high quality TEM analysis.

The invention relates in particular to a method for preparing a sample for a transmission electron microscope (hereinafter abbreviated TEM), comprising the following steps:

providing a substrate comprising on a surface thereof a patterned area comprising pattern features defining a topography,

-depositing a protective layer on the patterned region,

-generating a sample in the form of a slice by removing material on either side of a thin slice of the substrate, the slice being oriented transversely to at least several features for visualizing said features by TEM.

Characterized in that the method further comprises, before the step of depositing the protective layer, a step of generating a contrast layer on the topography by locally depositing a layer of contrast material in at least one target area spaced apart from the patterned area, wherein the local deposition is performed by electron beam induced deposition applied only to the at least one target area, such that a portion of the contrast material is also deposited around the target area, thereby forming a conformal layer of contrast material on at least some features in the patterned area.

According to one embodiment, the features of the patterned region are formed of a polymer and the contrast material is a heavy metal, such as Pt.

According to one embodiment:

-the features are parallel lines defined by a given width, height and spacing,

the patterned area is an array of such lines, and

the at least one target area is located on one side of the array, spaced from the array in a direction transverse to the lines.

According to an embodiment, the contrast material is deposited in a single target area, and the thickness of the conformal layer decreases as a function of distance from the target area.

According to an embodiment, the contrast material is deposited in two or more target areas, and the conformal layer is at least partially formed by: a conformal layer formed as a result of depositing contrast material in two or more target areas is added.

The invention also relates to the use of electron beam induced deposition to deposit a layer of contrast material over a patterned region comprising features of a pattern defining a topography by locally depositing the layer of contrast material in at least one target region spaced from the patterned region such that a portion of the contrast material is also deposited around the target region, thereby forming a conformal layer of contrast material over at least some of the features in the patterned region. The conformal layer is suitable as a contrast layer in the generation of TEM samples of patterned areas.

Drawings

Fig. 1 shows a front view and a top view of an array of polymer resist lines on a substrate.

Fig. 2 illustrates partial Pt deposition and conformal Pt layer deposition on the substrate of fig. 1 according to an embodiment of the invention.

Fig. 3a and 3b show details of the conformal layer on the array of resist lines before and after deposition of the spun-on carbon layer for the case of a single Pt deposition to one side of the resist line.

Figure 4 shows an embodiment in which two partial depositions are performed, one on each side of the array of resist lines.

Fig. 5a and 5b show details of the conformal layer on the array of resist lines before and after deposition of the spun-on carbon layer for the case of two Pt depositions.

Detailed Description

The preferred embodiment of the present invention will be described for the case of a set of parallel lines of polymer resist. The materials and processes known per se are mentioned only as examples and are not intended to limit the scope of the invention. Fig. 1 shows a substrate 1, which may be a glass substrate, having a silicon layer 2 on its surface. On the Si layer is a patterned region 8 comprising an array of parallel polymer resist lines 3, the patterned region being generated by lithographic patterning techniques known per se in the art. The pitch of the array of wires is of the same order of magnitude. The goal is to obtain TEM samples that allow validation of these dimensions. To this end, a protective spin-on carbon (SoC) layer is deposited on the resist line 3, and TEM samples of the substrate are produced in a Focused Ion Beam (FIB) tool by milling away material on either side of the lamella oriented in a direction perpendicular to the line 3. The profile of sample 4 is indicated in the top view of fig. 1. However, according to the present invention, additional steps are performed before the SoC layer is deposited.

As shown in fig. 2, a platinum layer 5 having a thickness T is locally deposited into a rectangular target area 6 on one side of an array of resist lines 3, spaced from the array by a distance D extending in a lateral direction with respect to the lines 3, in this case perpendicular to the lines. The local deposition is preferably done by Electron Beam Induced Deposition (EBID) in the FIB tool used to generate the TEM sample. The EBID technique is known per se and the details of the technique are not described here. When EBID deposition is limited to a given target area 6 located at a distance D from the patterned area 8 comprising the line 3, a thin layer of deposited material 7 is also created in the area around the target area 6. This thin layer is the result of the generation of secondary and backscattered electrons in the polymer material of the wires 3 as well as in the deposited material itself. By judicious choice of the distance D, the thickness T of Pt in the target area 6 and the deposition parameters applied in the EBID process, a thin layer 7 is formed conformally on the resist lines 3, i.e. this layer follows the topography defined by the lines 3 and does not fill the space between two adjacent lines 3.

When the material of layer 5/7 does not react with the polymer (as is the case with Pt), the conformal layer 7 does not damage the polymer line 3 given the fact that: the conformal layer 7 is formed outside the area 6 directly affected by the EBID process. As seen in fig. 2 and in more detail in fig. 3a, the conformal layer 7 has a thickness of a few nanometers, which gradually decreases as a function of distance from the target region 6. Preferably, the distance D and thickness T are chosen as a function of the dimensions of the array of lines 3 (height and width of the lines and pitch of the array) so that all lines 3 in the array receive a contrast layer detectable by the TEM. The parameters D and T, as well as other deposition parameters, may thus depend on the exact dimensions of the patterned area and the features within that area. However, a limited number of experiments are sufficient to find a suitable set of deposition parameters.

As seen in fig. 3b, a spun-on carbon layer 10 is then deposited on top of the Pt layer 7 to serve as a protective layer required during TEM sample processing. The protective layer may be another suitable material known in the art, applied by any known technique for this purpose. The substrate is then moved back into the FIB tool to produce the TEM sample 4. The TEM image that can be taken from sample 4 corresponds to the cross-sectional view shown in fig. 3 b. Even if the conformal layer 7 does not have a constant thickness, it provides a clear contrast between the lines 3 and the SoC layer 10 and thereby permits the lines 3 to be clearly visualized in the TEM image such that their dimensions can be measured and/or verified. Furthermore, the Pt layer 7 protects the polymer line 3 from any damage during deposition of the SoC layer 10. The deposition by EBID is carried out only on the target region 6, i.e. not directly on the region of interest 8, thereby avoiding possible damage to the polymer lines 3 caused by the high electron current applied in the EBID process.

Fig. 4 shows an embodiment in which local Pt layers 5a and 5b of equal thickness T are deposited in two equally sized

rectangular target regions

6a and 6b on either side of a patterned region 8 comprising an array of polymer resist lines 3, the two target regions being placed at equal distances D from the array. The

layers

5a and 5b are applied sequentially, i.e. by EBID deposition in the area 6a and subsequently in the area 6b (or vice versa). The reduced thicknesses of the conformal Pt layers 7a and 7b resulting from the two Pt depositions now add up and form a contrast layer having a substantially constant thickness, as shown in the detail images in fig. 5a and 5 b. The image acquired from TEM sample 4 is now similar to the view shown in fig. 5 b. The contrast layer 7a +7b has a substantially constant thickness across the array of resist lines 3.

However, by depositing two

Pt layers

5a and 5b having a lower thickness than in the example shown, or further away from the array of resist lines 3, the combined conformal layer 7a +7b can have a higher thickness on the outer lines than in the middle of the array, however this lower thickness is sufficient to provide the required contrast. Layer 5a may also be deposited at a different distance from array 8 than layer 5b, for example where the available space for the target area is different on both sides of the array. In this case, the thicknesses of the

layers

5a and 5b may be different to ensure that a conformal layer of the appropriate thickness is ultimately formed on the lines 3. More than two

layers

5a, 5b, 5c, … … may be deposited sequentially in more than two

respective target regions

6a, 6b, 6c, … … if so desired for the pinning and other characteristics of the patterned region. The in-plane shape of the target 6 or

area

6a, 6b, … … may be different from the rectangular shape shown in the various figures. If the contrast layer 7 (or 7a +7b + … …) is only required on certain features in a sub-region of the patterned region 8, the thickness T and/or the distance D, and possibly other parameters, may be adapted such that the contrast layer 7 is deposited at least on said sub-region of the patterned region 8. The method of the present invention thus allows a degree of flexibility, which is a function of the nature of the structure of the TEM sample in which it is required.

Numerical example:

the following EBID parameters were applied to obtain a Pt contrast layer on a polymer resist line array like the one shown in each figure, with a line width measured perpendicular to the longitudinal direction of the line of about 14nm, a height of about 15nm and a pitch of about 30 nm.

Primary beam energy: 5keV

Current through the aperture: 1.6nA

Deposition time: 60s

Distance D (fig. 2): 2 to 4 m

Temperature: at room temperature

Thickness T (fig. 2): 2 to 20nm

In-plane size of Pt target area 6: typically 0.3m x 0.2m but may be selected depending on local structural features.

The present invention is not limited to any of the materials described above. The invention is primarily intended for producing TEM samples comprising the following characteristics of the materials: a delicate material such as polymer or porous silicon and/or a material that shows little or no contrast with the protective layer as required for TEM sample preparation. The contrast layer may be formed of any material that does not react with the material of the feature to be imaged by the TEM. For imaging polymer structures, other heavy metals than Pt are suitable as materials for the contrast layer, e.g. W, Hf, Mo, Au, Ir, … … which can be deposited in the FIB tool using suitable chemical precursors and EBID modes.

The structure to be imaged may be any patterned structure defined by a given topography. The invention is applicable to all scaled structures and stacks where the top is resist, as used in patterning, or structures with a complex resist layer as used in DSA (directed self assembly), SADP/SAQP (self aligned double and quadruple patterning) methods, or structures where the polymer activation layer needs to be analyzed for selective deposition, for example.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims shall not be construed as limiting the scope.

Claims

1. A method for preparing a sample for transmission electron microscopy, TEM, comprising the steps of:

providing a substrate (1, 2) comprising a patterned region (8) on a surface thereof, the patterned region (8) comprising pattern features (3) defining a topography,

depositing a protective layer (10) on the patterned region (8),

producing the sample in the form of a thin slice (4) of the substrate by removing material on either side of the slice, the slice being oriented transversely to at least a number of the features (3) so as to visualize the features by TEM,

characterized in that the method further comprises: -a step of generating a contrast layer (7) on the topography by locally depositing a layer of contrast material (5) in at least one target area (6) spaced apart from the patterned area (8) prior to the step of depositing the protective layer (10), wherein the local deposition is performed by electron beam induced deposition applied only to the at least one target area (6) such that a portion of the contrast material is also deposited around the target area (6), thereby forming a conformal layer (7) of the contrast material on at least some of the features in the patterned area (8).

2. The method of claim 1, wherein the features (3) of the patterned region are formed of a polymer, and wherein the contrast material is a heavy metal, such as Pt.

3. The method of claim 1, wherein:

said features being parallel lines (3) defined by a given width, height and spacing,

the patterned region (8) is an array of such lines, and

the at least one target region (6) is located on one side of the array, spaced from the array in a direction transverse to the line (3).

4. The method of claim 1, wherein the contrast material is deposited in a single target region (6), and wherein the thickness of the conformal layer (7) decreases as a function of distance from the target region (6).

5. The method of claim 1, wherein the contrast material is deposited in two or more target areas (6a, 6b) and the conformal layer is at least partially formed by: adding a conformal layer (7a, 7b) formed as a result of the contrast material to be deposited in the two or more target regions (6a, 6 b).

6. Using electron beam induced deposition to deposit a layer of contrast material on a patterned region (8) comprising topographical defining pattern features (3) by locally depositing the layer of contrast material (5) in at least one target region (6) spaced from the patterned region (8) such that a portion of the contrast material is also deposited around the target region (6), thereby forming a conformal layer (7) of the contrast material on at least some features in the patterned region (8).